Little is known about how behavior change is mobilized and sustained in AA, and even less is known about how co-occurring psychiatric severity may mediate such changes. This is remarkable given that Twelve-step (TS) therapy is the prevailing alcohol treatment model in the United States and a majority of persons receiving alcohol treatment (TS and non-TS) will attend AA, if only for a limited time. Significant gains have been made in identifying the: (1) characteristics of people most likely to attend AA, and (2) understanding the nature and magnitude of benefits associated with AA exposure. What actually occurs in AA, and how these specific behaviors may predict improved functioning, however, remains poorly understood and rarely investigated. The overall aim of this study is to test a transtheoretical model of behavior change in AA, and to identify the specific AA-related behaviors that mobilize such change. This application proposes a 2-year longitudinal study (N = 300) of AA-exposed individuals, with participants recruited from AA and out-patient treatment. The study has five aims that prospectively test how prescribed AA-related behaviors mobilize AA-specific and non-specific change mechanisms, and how such mechanisms once mobilized predict reductions in drinking and other drug use. A theme in this application is to argue for ecologically sound AA studies, but not at the expense of sacrificing rigorously designed and executed science. To this end, AA-related behaviors, behavior change mechanisms, and substance use are each defined multi-dimensionally, using assessments with strong psychometric properties. New knowledge of the general and mediational processes of behavior change in AA is critical for improvement of TS-based therapy. Study findings will provide TS oriented therapies with evidenced-based feedback that is instrumental for improving intervention strategies.